Storage of recently used 1st packets added. Connections with the same
1st packet are disallowed

- one with just middle-server implementation
- one with proxy config mock and multi-server DC setup

And make it possible to make proxy connect to it

If downstream connection pool crashed and recovered while some
upstream was alive holding it's DC ID, it may return downstream
taken from the old incarnation of pool to new one because it's
registered under the same name.
Fix is to hold pool's PID instead of name